A polyester is a polymeric material which can be made from the esterification of polybasic organic acids with polyhydric acids. Perhaps the most commonly made and used polyester is polyethylene terephthalate (PET), which can be manufactured by reacting terephthalic acid with ethylene glycol.
Polyesters are currently being used in increasing amounts in various applications. For instance, polyesters are commonly used to make all types of containers such as beverage and food containers, photographic films, X-ray films, magnetic recording tapes, electrical insulation, surgical aids such as synthetic arteries, fabrics and other textile products, and other numerous items.
The formation of polyesters such as PET typically involves polymerization at high temperatures and under high vacuum conditions. Polymerization typically involves a two-step process, i.e., polymerization to form PET followed by "solid-stating" of the PET.
Polymerization of monomeric materials to form PET also provides certain side products including DEG and acetaldehydes. The solid-stating of the PET can provide for a reduction of these side products, e.g., acetaldehydes. Solid-stating also increases the molecular weight of the polymer material.
In addition to the challenges of polymerization, another significant issue in the field of polyester technology involves the recovery and recycling polyesters. Because polyesters can be economically remelted and reformed, many efforts are underway to recycle as much polyester as possible after use. Before polyesters can be recycled, however, it is necessary to separate the "post-consumer" polyesters from other products and materials that may be found mixed with or attached to the polyester. Unfortunately, many problems have been encountered in attempting to separate polyester from other waste materials. In particular, many prior art processes are not capable of efficiently or economically recovering polyester when a significant amount of other material, impurities, and contaminants are present.
Many prior art processes for separating polyesters from other materials have focused on "floatation" separation techniques and mechanical recovery processes. In floatation separation, polyesters are separated from other materials based on density differences. For instance, materials containing polyester can be combined with water. The less dense materials that float in water can thus be easily separated from the submerged polyester. This procedure is relatively simple and can be effective in separating polyesters from many low density impurities. Floatation separation techniques, however, cannot be used if the polyester is found in combination with materials that sink in water or that have densities comparable to that of polyester.
Examples of the latter include polyvinyl chloride (PVC) and aluminum, which are not water buoyant. In fact, PVC has a density that is very similar to the density of PET and is often misidentified as PET. Both aluminum and PVC must be separated from polyester before it can be reused. In particular, if PET and PVC are remelted together, hydrochloric acid gases are produced which destroy the properties of the resulting plastic material.
Besides failing to separate polyesters from heavier-than-water impurities, floatation separation techniques and conventional washing also fail to remove coatings or other contaminants that are commonly adhered to polyester. For example, polyester containers are commonly coated with vapor barrier coatings, saran coatings, and/or inks.
Mechanical recovery processes typically involve washing processes used to strip surface coatings and contaminants off polyester without any substantial reaction occurring between the polyester and the wash solution. For example, U.S. Pat. Nos. 5,286,463 and 5,366,998, both of which are incorporated herein by reference in their entireties, disclose a composition and process for removing adhesives, particularly polyvinylidene halide and polyvinyl halide based resins, from polyester films, such as photographic films. In one embodiment, the polyester films are mixed with a reducing sugar and a base to remove the adhesive polymeric resin from the film. An acid is then added to precipitate the resin that can then be separated from the polyester film.
Recently, the focus of recovering polyester from the waste stream has turned to chemically converting the polyester into usable chemical components. Alkaline materials have been employed in such processes. For instance, in U.S. Pat. No. 5,395,858 and in U.S. Pat. No. 5,580,905, both of which are incorporated herein by reference in their entireties, disclose processes for recycling polyesters in which the polyesters are reduced to their original chemical reactants. The process includes the steps of combining the polyester materials with an alkaline composition to form a mixture. The mixture is heated to a temperature sufficient to convert the polyester to an alkaline salt of a polybasic organic acid and a polypol. During the process, the alkaline composition is added in an amount sufficient to react with all polyester present in the mixture.
The foregoing process provides for the complete chemical conversion/saponification of the polyester material. However, this can add a substantial cost to the overall process since the polyester must ultimately be reformed. Accordingly, a technique that only partially saponifies the polyester has been developed. This process is discussed in copending U.S. application Ser. No. 08/631,710 which is incorporated by reference in its entirety for all purposes.
In this copending application, the partial saponification of the polyester is provided by a process which includes the steps of combining polyester with an alkaline solution to form a mixture. The alkaline composition is preferably added to the materials in a stoichiometric amount sufficient to react with up to about 10% of the polyester. The mixture is then heated to a temperature sufficient to saponify a portion of the polyester but insufficient to melt the polyester. This heating and saponification process allows for the removal of a variety of surface contaminants and absorbed impurities including coatings and dirt adhered to the polyester, and organic and inorganic compounds entrained within the polyester.
Despite the ability of this recovery process to impurities and contaminants from polyester, the art is continuing to look for ways to make the recovery process more cost effective and provide a superior recycled product.